XZ gantry play - troubleshooting

Hi I have had issues with some level of play in the XZ gantry. If I pull and push on the base of the router I can see approximately 1/8” of movement in the tip of the end mill when compared to the grid on my spoilboard.

It is a Longmill MK1 I purchased used and have put probably about 30 hours of use on in past month.

Steps I have done so far in order to correct:

  1. All backlash nuts loosened and then re-tightened, machine tested so that it moves evenly in all axis.

  2. Delrin V wheels in all axis adjusted so difficult to turn by hand and approximately even in turning difficulty by adjusting eccentric nuts.

  3. Z axis linear rail screws tightening.

  4. Tension on all bolts holding X rail and Y rails to PLA supports and each other checked and tightened.

  5. Check tension on bolts that hold the two aluminum pieces making up the X axis together.

  6. Motor housing bolts on top of XZ gantry tightened.

The machine’s performance is satisfactory on softwood and hardwood but I would like to experiment on milling aluminum within next few months and would like to be able to move up a bit on my feed rate to decrease cut times. At faster speeds I have noticed the machine vibrates a bit with more aggressive cuts - 1/4 end mill profile cuts (full bit engagement, pass depth 0.125) on maple at 55 ipm.

Steps I am considering:

  1. Re-squaring Longmill to work table - it is difficult to get the eccentric nuts on the right sided rail of the Y gantry to tighten evenly. The top rear wheel in particular looks like it may be off center by a mm. My machine is out of square by 0.5-2mm when I used the Gsender alignment functions

  2. If the above does not resolve, replace delrin V wheels.

  3. Tramming router. Was planning on doing this after a work bench overhaul this winter.

Any other suggestions to decrease or eliminate play in the XZ gantry are appreciated. If I have time next week and I am unable to resolve I will call Sienci and get someone to walk through what I should be checking.

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Update - figured I’d post this so others can learn from my experiences.

Spend the past several hours tinkering.

  1. XZ gantry dismantled. On reassembly I identified a major source of play - the lead screw lock nut wasn’t tightened. Timing belt also aligned better. The hum of the Z gantry jogging now sounds very crisp without unusual vibrations.

  2. The bushing at the front right end was damaged. It was like this from when I purchased but I wasn’t that familiar with the machine or CNC in general - the little plastic piece that holds the bearings had been floating around on the lead screw for a while.

  3. Mounting surface is warped, which is why the one Y axis gantry is more difficult to get the eccentric nuts and delrin wheels to behave. I had been planning on building a torsion table this winter to mount it on but I may have to get some plywood and a new spoiler board in the mean time.


Welcome to the forum Mike! Glad you identified you problem and shared with us. I read your original post but couldn’t think of anything to check that wasn’t already on your list.

That post had me concerned… In one of their Q&A videos, Andy & Chris had brought up the issue of bolts loosening up and to make sure to keep a good maintenance cycle.

I don’t know if you have the MK1 or MK2, [ update: facepalm on the forehead… that is the first thing you mention! ] … they also mentioned that the MK2 comes with an improvement that includes lock washers, and soft washers that reduce the chances for such issues.

I do regularly maintain the machine - I have a flow that involves per job checks and cleaning as well as weekly checks. Part of the issues I am running into is I am new to CNC and bought the machine already mounted to a table from a friend - table discovered yesterday to not be flat and level (facepalm). I’ve read the assembly manual end to end and the regular maintenance tips but for some reason missed the notes on checking the motor / lead screw couplers and the ACME nuts.

Part of the issue with the Z axis lead screw was that the ACME nut was only loose by a very small amount which I didn’t notice until I removed and reinstalled the lead screw and after it was snug the Z axis play vanished.

I believe one of the upgrade options for the 48x30 MK1 is to include an adapter MK2 rail and XZ gantry, but that is likely not in the budget this year, largest carve jobs have only been 13” as of yet.

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